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Date: Sat, 21 Feb 2015 16:42:58 +1100
Subject: Re: Design thought for callbacks
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:86016

On Sat, Feb 21, 2015 at 1:44 PM, Cem Karan <cfkaran2@gmail.com> wrote:
> In order to inform users that certain bits of state have changed, I requi=
re them to register a callback with my code.  The problem is that when I st=
ore these callbacks, it naturally creates a strong reference to the objects=
, which means that if they are deleted without unregistering themselves fir=
st, my code will keep the callbacks alive.  Since this could lead to really=
 weird and nasty situations, I would like to store all the callbacks in a W=
eakSet (https://docs.python.org/3/library/weakref.html#weakref.WeakSet).  T=
hat way, my code isn't the reason why the objects are kept alive, and if th=
ey are no longer alive, they are automatically removed from the WeakSet, pr=
eventing me from accidentally calling them when they are dead.  My question=
 is simple; is this a good design?  If not, why not?  Are there any potenti=
al 'gotchas' I should be worried about?
>

No, it's not. I would advise using strong references - if the callback
is a closure, for instance, you need to hang onto it, because there
are unlikely to be any other references to it. If I register a
callback with you, I expect it to be called; I expect, in fact, that
that *will* keep my object alive.

ChrisA